Method and device for producing a powder aerosol and use thereof

ABSTRACT

Powder aerosols are introduced into a gas stream in metered form and without any aggregates. To this end, according to the invention, a powder is drawn, for example by mechanical device, from a reservoir area and is converted into a powder aerosol which is introduced within a flowing gas stream into a container to which ultrasound is applied. The ultrasound breaks down powder lumps and aggregates, and the aerosol can be fed in a uniform, fine powder distribution within the gas stream to a plasma burner. Apparatus for carrying out this invention includes a feed tube ( 4 ) connected to a mechanical powder feed device ( 6 ), so that an aerosol containing the powder enters a vessel ( 10 ) containing the ultrasound horn ( 2 ). The method and the associated apparatus are used for the coating of components, preferably for applying a protective coating to a turbine blade.

BACKGROUND

1. Field of the Invention

The invention relates to a method for producing a powder aerosol, whichcan be fed in metered form via a gas stream to a plasma burner, at aconstant mass flow rate, without any aggregates. In addition, theinvention also relates to an apparatus for producing a powder aerosolhaving a container which is connected via an ultrasound horn to anultrasound transmitter and which has a feed tube and an outlet tube.

2. Related Art

A high-power plasma burner, with radio-frequency excitation, can be usedto completely vaporize substances which are in the form of powder, in ashort time, and then deposit, from the vapor phase, a coating with apredetermined composition onto a substrate, with a high growth rate. Afirst application for this method, which is known as plasma flashevaporation, is the evaporation of fine-grained powder composed ofyttrium-barium-copper oxide, with subsequent deposition of ahigh-temperature superconductor layer (HTSL). Another major applicationis the coating of components, for example the application of aprotective coating to a turbine blade.

In order to propel the powder into the plasma, the powder is firstintroduced into a propellant gas, so that an aerosol is produced whichcan be passed into the plasma burner. The aerosol must be free ofaggregates of powder particles which cannot be vaporized, that is to sayaggregates formed from individual particles. Preferably, the powder feedprocess is reproducible and uniform, that is to say free of variationswith time.

Various methods, associated with the general plasma spraying technique,are known for producing the aerosol required for plasma flashevaporation. For example, a disk feed device operates in such a way thatan amount of powder which is defined by the dimensions of a meteringgroove is deposited continuously, by means of the metering groove, on aslowly rotating disk. The powder, which revolves with the disk, is movedby a wiping apparatus to the edge of the disk where it falls into aretaining apparatus, where it is retained within a flowing gas stream.In the case of powders whose grain size is very fine, for example in arange equal to or less than 20 μm, the dispersion of the powder ishighly restricted by the forces which act between the particles. Thisleads to aggregates with dimensions in the millimeter range entering thegas stream at certain time intervals rather than a continuous flow ofdiscrete, individual powder grains, which are separated from oneanother, as required. Even if the aggregates are broken down within thegas stream during transportation to the plasma burner, the lumpystructure of the introduced powder results in fluctuations in the massflow rate which are not acceptable, that is to say a variation in theamount of powder introduced into the plasma over time. Screw-type orworm-type feed devices, which are also known from the prior art, and inwhich the powder is fed by a worm gear to a mixing area where it entersthe propellant gas stream, have the same disadvantage.

In the case of a brush metering device, the powder, which is formed intoa plug in the form of a cylindrical body, is pushed forward through atube to a rotating metal brush which removes from the plug body anamount of powder proportional to the feed rate. A gas stream passingthrough the brush area picks up the powder and transports it to thepoint at which it is required. Experience has shown that the process ofrubbing the powder off the plug body results in aggregates which cannotbe vaporized.

WO 96/03216 A1 discloses a powder feed device which operates on theultrasound principle. In this case, the powder to be fed has ultrasoundapplied to it in a vessel in which it is also stored, is swirled, itsaccumulations are broken down, and it is picked up by a gas flow passingthrough the vessel. Experiments have shown that the feed process isreproducible and is free of fluctuations. This method is limited by thefact that only a limited amount of powder can be stored and fed. Inparticular, the feed rate, that is to say the mass fed per unit time,can in this case not be adjusted independently of the gas flow rate,since the gas flow also contributes to the swirling of the powder, inaddition to the effect of the ultrasound.

Against this background, the object of the invention is to improve thepreparation of the aerosol and to provide an associated apparatus and tospecify a suitable application.

SUMMARY OF THE INVENTION

In accordance with the invention, mechanical means are used to convert apowder to an aerosol. For the purposes of the invention, a gas stream isfed with the powder aerosol to a plasma burner through a storage orbuffer means to which ultrasound can be advantageously applied.

The invention assures the powder aerosol is present in the gas streamwith a uniform, fine particle distribution. In the process, the powderaerosol, which has been treated in this way, uniformly has itsaggregates broken down, and any initial fluctuation in the flow rate ofthe powder is stabilized.

In a preferred embodiment apparatus for carrying out the inventionemploys a feed tube connected to a mechanical powder feed device thatdispenses the powder within a gas stream flowing within the feed tube.The feed tube is connected at a downstream end to a storage and/orbuffer vessel to which ultrasound is applied A vessel outlet conduitconveys the gas stream to the plasma burner. The powder aerosol entersthe volume formed by the ultrasound vessel, is made more uniform by theeffect of the ultrasound, and leaves the ultrasound vessel as a moreuniform and continuous aerosol flow and is fed to a plasma burner. Inthis case, the vessel is preferably closed at the top by a cover. Inparticular, the vessel may at the same time contain the ultrasound horn.

The invention thus makes it possible for a very fine powder to beintroduced into a plasma in any desired amount continuously, withoutaggregates, reproducibly and without any variation in the mass flow rateof the powder. This is achieved by the combination of a disk, worm orbrush feed device using the ultrasound principle on which WO 96/03216 A1is based.

The invention is used for the coating of components and is particularlysuitable for applying a protective coating to turbine blades. In thiscase, it is particularly advantageous that suitable coating thicknessescan be deposited in reasonable times due to the slight relative movementwhich is possible between the apparatus according to the invention andthe turbine blades.

BRIEF DESCRIPTION OF THE DRAWINGS

Further refinements and advantages of the invention result from thefollowing description of a FIGURE of an exemplary embodiment, based onthe single FIGURE in the drawing, in conjunction with the patent claims.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The figure shows an exemplary embodiment of a powder feed systemaccording to the invention, which is intended for operation of a plasmaburner. The plasma burner is an induction plasma generator 11 which isexcited at radio frequency and forms an inductively coupled plasma, witha directed plasma flame 12, generated from a fuel gas. An aerosol whichis completely vaporized owing to the high plasma temperatures isintroduced into the plasma flame 12. This makes it possible to depositmaterials from the vapor phase onto component surfaces, with a highgrowth rate.

The invention is intended to be used to apply protective coatings toturbine blades. A portion of a curved turbine blade 13 is indicated inthe figure, including the coating 14. Such turbine blades are subject tosevere stresses in conjunction with cycling temperatures duringoperation, for which reason the blades, which are made from specialalloys, are coated. By way of example, coatings composed of ceramic, inparticular zirconium oxide (ZrO₂), or else other oxidic or nitridicmaterials, may be used. The coating materials may also be interchangedin induction plasma burners without any major effort, so thatmulticoating systems can easily be produced.

An ultrasound horn 2, which is connected to an ultrasound generator 1 inorder to generate ultrasound at a predetermined frequency, is designedpartially in the form of a hollow body in order to transmit theultrasound at a suitable wavelength, and forms a container to whichultrasound can be applied. A “cup-type ultrasound horn” is thus definedas the ultrasound vessel 10, and can be closed by a cover 3. The cover 3has a feed tube 4 and an outlet tube 5 passing through it, with the feedtube 4 and the outlet tube 5 being arranged symmetrically with respectto one another.

In an alternative embodiment, a separate ultrasound horn 2 can beconnected, for ultrasound purposes, to a separate vessel.

A powder feed device 6, which may be a mechanical disk, worm, brush orother feed device which is suitable for the metering of very finepowders and is shown in the figure, is used to introduce a powder P,which is stored in a container 7, into a gas stream of a propellant gasT, thus producing a powder aerosol A.

The powder feed device 6 is connected to the cup-type ultrasound horn 2in such a way that the powder aerosol A produced by it is passed throughthe feed tube 4 into the interior of the cup-type ultrasound horn 2,where it floats in the gas stream. If ultrasound were not applied,relatively heavy particles, such as powder lumps or aggregates, would inthis case be deposited on the bottom of the vessel 10.

The affect of the ultrasound is to breakdown the powder lumps oraggregates, which has led to problems in the past. A powder aerosol A′,which is processed with a desired, uniform, fine particle distribution,leaves the cup-type ultrasound horn 2 through the outlet tube 5, and canbe fed in the gas stream to the plasma burner 11.

The specific combination of the powder feed device 6 and the ultrasounddevice which has been described in detail, above, advantageously resultsin the unexpected affect of the powder flow being made uniform. Theprocess of making the powder flow uniform can be explained by thestorage and/or buffer affect of the volume in the vessel, to which theultrasound is applied, with the powder aerosol A. This results in aparticularly high quality coating 14 being produced by the plasma burner11 on a substrate, in particular on a turbine blade 13.

What is claimed is:
 1. A method for producing a powder aerosol which canbe fed in metered form via a gas stream to a plasma burner, for whichpurpose the powder aerosol without any aggregates is required to beprovided at a constant mass flow rate, comprising the following methodsteps: drawing a powder from a reservoir area by mechanical means;converting the powder, via a propellant gas, to the powder aerosol;introducing the powder aerosol into a container to which ultrasound canbe applied and whose volume to which ultrasound is applied is used as astorage and/or buffer means for the aerosol; using the ultrasound affectto break up powder lumps or aggregations in the powder aerosol, thusmaking the powder aerosol more uniform and finer; and feeding the powderaerosol, which has been made more uniform and finer, to the plasmaburner.
 2. The method of claim 1 including the step of applying thevaporized powder aerosol produced by the plasma burner to a turbineblade as a protective coating.
 3. The method claimed in claim 2, whereinthe protective coating is a ceramic coating.
 4. The method claimed inclaim 2 wherein the coating is composed of zirconium oxide (ZrO₂).
 5. Anapparatus for producing a powder aerosol, comprising a container whichis connected to an ultrasound transmitter via an ultrasound horn andwhich has a feed tube and an outlet tube, in which case the feed tube isconnected to a powder feed device which includes connections for areservoir container for a powder and for a propellant gas, and with theoutlet tube being connected to a plasma burner.
 6. The apparatus asclaimed in claim 5, in which the ultrasound horn includes a vessel inthe form a cup.
 7. The apparatus as claimed in claim 6, wherein the cupforms the container.
 8. The apparatus as claimed in one of claim 5wherein the container is closed at the top by a cover.
 9. The apparatusas claimed in claim 5 wherein the feed tube and the outlet tube arearranged symmetrically with respect to the ultrasound horn or thecontainer to which ultrasound is applied.
 10. The apparatus as claimedin claim 5, wherein the plasma burner includes an induction plasmagenerator.
 11. The apparatus of claim 2 wherein the vaporized powderaerosol produced by the plasma burner is applied as a protective coatingto a turbine blade.
 12. The apparatus of claim 11 wherein the coating isceramic.
 13. The apparatus of claim 11 wherein the coating is zirconiumoxide (ZrO₂).